In non-coherent wireless communications, the phase of the transmitted electrical field may not be used to carry information. Instead, properties such as frequency, amplitude, or intensity are used by the transmitter to send information. This type of communication finds applications in areas where modulation of the phase may not be feasible. This could occur for several reasons. For instance, some communication links may suffer from severe degradation due to the existence of fast multipath fading during transmission. This degradation caused by large delays in the multipaths can completely destroy phase information at the receiver, making coherent communication impossible. One example of this is underwater communications using radio frequency (RF) or other spectrum. Underwater communication may suffer from severe delay and Doppler spreading effects: the delay caused by reflections from objects such as rocks and the Doppler spreading is caused by waves. Non-coherent modulation schemes such as frequency shift keying (FSK) have traditionally been used in this type of application.
Another use for non-coherent wireless communication is in the area of optical wireless communication. The need for higher data rates in wireless communication systems and the scarcity of available unlicensed bandwidth have fueled research into the use of spectrum outside of the RF band for wireless communication. Optical wireless communication such as UV, visible light, and infrared may provide a potential for very large unlicensed bandwidth that will satisfy the bandwidth needs for current and future wireless communication applications. Typically, first generation optical wireless communication systems may need to be non-coherent. This is due to the complexity and cost of coherent optical modulators capable of modifying the phase of the optical radiation, and also due to the difficulty of recovering phase information at the receiver due to large scattering and reflections exhibited by optical radiation in the presence of air molecules and aerosols.